1) Field of the Invention
The present invention relates to a mobile communication system for digital automobile telephones or the like. Particularly, the present invention relate to a mobile communication system for digital automobile telephone services, a communication control method for a mobile communication system, a base station in a mobile communication system, and a mobile station in a mobile communication system.
2) Description of the Related Art
In general digital automobile telephone systems, concepts about communication level and communication out level have been employed.
The communication level means a receiving level at which waiting is permitted so that the receiving level of each mobile station allows communications to a base station without any trouble.
The communication out level means a receiving level which is judged that a mobile station is out of communicable zone when the receiving level of the mobile station in communication state is less than a communication out level.
A base station transmits only one communication level and one communication out level. If a mobile station has a receiving level more than a communication level, it is displayed that the mobile station is in a communicable zone, thus performing originating and destination calls. If the receiving level is not less than a communication out level, the mobile station once entering a communicable zone can perform originating and destination calls. If the receiving level is less than a communication out level, it is displayed that the mobile station is out of a communicable zone so that the originating and destination calls cannot be established.
In other words, the mobile station within the area for a base station depends on only a down receiving level value (or judged only at a down receiving level) to judge whether the receiving level is a communication level or communication out level level. Hence the up receiving level (the receiving level received by a base station while a mobile station originates) is not used as a communication control judgment.
In the current mobile communication systems, it cannot be assured that the communication qualities in up and down directions are well-balanced in view of system design (line design). The transmission output of a mobile station depends on a variety of transportable machine, portable machine, and mobile stations.
Now let us consider that the transmission output of a mobile station is considerably smaller than that of the base station. Even if the down link (from a base station to a mobile station) has a receiving level sufficient in the connection or communication quality, a question in a connection quality such as an erroneous connection of an originating signal may occur or a question in communication quality such as a bad up-link connection may occur, because of the small transmission output of the up link (from the mobile station to the base station).
Let us consider the cases, for example, shown in FIG. 24. When the base station has its transmission power of 1.0 W (electric field intensity 30 dBm: 143 dB.mu.V) and its propagation loss is 135 dB, the receiving level of the mobile station is 8 dB.mu.V, thus satisfying the desired quality of the down transmission (1).
In consideration of the up transmission (2) with the transmission output of the mobile station of 3.0 W (electric field intensity 34.7 dBm: 147.7 dB.mu.V), the receiving level of the base station is 12.7 dB.mu.V, whereby the desired quality in an up link is satisfied.
In consideration of the up transmission (3) with the transmission output of the mobile station of 2.0 W (electric field intensity 33.0 dBm: 146.0 dB.mu.V), the receiving level of the base station is 11.0 dB.mu.V, whereby the desired quality in an up link is satisfied.
In consideration of the up transmission (4) with the transmission output of the mobile station of 0.8 W (electric field intensity 29.0 dBm: 142.0 dB.mu.V), the receiving level of the base station is 7.0 dB.mu.V, whereby the desired quality in an up link is not satisfied.
In consideration of the up transmission (5) with the transmission output of the mobile station of 0.3 W (electric field intensity 24.7 dBm: 137.7 dB.mu.V), the receiving level of the base station is 2.7 dB.mu.V, whereby the desired quality is not satisfied.
As described above, there are the case where the transmission output of the mobile station side satisfies the quality in an up link and the case where the transmission output of the mobile station side does not satisfy the quality in an up link.
Next, the case shown in FIG. 25 is considered. With the transmission output of the base station of 3.0 W (electric field intensity 34.7 dBm: 147.7 dB.mu.V) and the propagation loss of 139.7 dB, the receiving level of the mobile station is 8 dB.mu.V, whereby the desired quality in the down transmission (6) is satisfied.
In consideration of the up transmission (7) with the transmission output of the mobile station of 3.0 W (electric field intensity 34.7 dBm: 147.7 dB.mu.V), the receiving level of the base station is 8.0 dB.mu.V, whereby the desired quality is satisfied in an up link.
In consideration of the up transmission (8) with the transmission output of the mobile station of 2.0 W (electric field intensity 33.0 dBm: 146.0 dB.mu.V), the receiving level of the base station is 6.3 dB.mu.E, whereby the desired quality is not satisfied in an up link.
In consideration of the up transmission (9) with the transmission output of the mobile station of 0.3 W (electric field intensity 24.7 dBm: 137.7 dB.mu.V), the receiving level of the base station is -1.9 dB.mu.V, whereby the desired quality is not satisfied in an up link.
As described above, even when the transmission output of the base station is as high as 3.0 W, there are the case where the transmission output of the mobile station satisfies the quality in up link and the case where the transmission output of the mobile station does not satisfy the quality in up link.